Cellular immunity mediated by nonspecific cytotoxic cells (NCC) in catfish and natural killer cells (NK) in mice and humans will be analyzed and compared. This unique approach will be accomplished with monoclonal antibodies (MAbs) which have been derived against target cell antigens involved in the recognition function of NCC and NK. These MAbs inhibit the lysis of target cells by both NCC and NK. Using these MAbs as biological probes studies will be performed to: (a) define the species distribution and expression of the target cell molecules; (b) examine the biological role of these structures in immune function; and (c) determine the similarity of these molecules to other known antigens. At the biochemical level, analyses will be made of the primary structure of these molecules (e.g., glycosylation). Studies at the molecular level will examine the regulation of these molecules and attempt to identify and isolate the gene(s) encoding these molecules. Studies proposed in this grant will contribute significant knowledge to the field of comparative immunology. More importantly, the studies will address a major question in NK cell biology: What molecules on the surface of target cells are recognized by NK cells? And what role do these molecules serve in normal immune function? To achieve these aims, the highly sensitive technique of flow cytometry in combination with various biochemical and molecular analyses will be utilized to study those molecules in fish, mice and humans. The ultimate goal of the project is to fully characterize a novel structure involved in immune function across species barriers.